Proteins in Food: An Introduction

Abstract

Proteins are the most abundant molecules in cells, making up 50 % or more of their dry weight. Every protein has a unique structure and conformation or shape, which enables it to carry out a specific function in a living cell. Proteins comprise the complex muscle system and the connective tissue network, and they are important as carriers in the blood system. All enzymes are proteins; enzymes are important as catalysts for many reactions (both desirable and undesirable) in foods.

For use with subsequent Protein food chapters.

Glossary

Amino acid

Building block of proteins; contains an amino group, a carboxyl group, a hydrogen, and a side chain, all attached to a central carbon atom.

Amphiphilic

A molecule that contains both hydrophobic and hydrophilic sections.

Amphoteric

Capable of functioning as either an acid or as a base depending on the pH of the medium.

Alpha helix

Ordered protein secondary structure: corkscrew shape, stabilized by intrachain hydrogen bonds.

Beta-pleated sheet

Ordered protein secondary structure; zigzag shape, stabilized by interchain hydrogen bonds.

Conformation

The specific folding and shape that a protein assumes in space.

Denaturation

Changes in the conformation (secondary, tertiary, or quaternary structure) of a protein caused by changes in temperature, pH, or ionic strength, or by surface changes.

Dipeptide

Two amino acids joined by a peptide bond.

Disulfide bond

Strong covalent bond formed by the reaction of two thiol (SH) groups.

Functional property

Characteristic of the molecule that enables it to perform a specific role in a food. Examples of functional properties of proteins include solubility, thickening, binding, gelation, foaming, and emulsifying capacity.

Hydrolysis

Breaking of one or more peptide bonds in a protein to form smaller polypeptide chains.

Hydrophilic

Water-loving; characteristic of polar and charged groups.

Hydrophobic

Water-hating; characteristic of nonpolar groups.

Isoelectric point

pI; the pH at which the overall charge on a protein is zero; the number of positive charges is equal to the number of negative charges; the protein is most susceptible to denaturation and precipitation at this pH.

Maillard browning

The free carbonyl group of a reducing sugar and the free amino group of a protein react to form a brown color; complex nonenzymatic reaction that is favored by high temperatures.

Peptide bond

Bond formed by the reaction of the amino group of one amino acid and the carboxyl group of another.

Polypeptide

Several amino acids joined together by peptide bonds.

Protein primary structure

Specific sequence of amino acids along the protein chain, joined by peptide bonds; the covalently bonded protein backbone.

Protein quaternary structure

The noncovalent association of protein chains to form a discrete unit.

Protein secondary structure

Three-dimensional arrangement of sections of the protein chain; secondary structures include the α-helix, β-pleated sheet, and random coil.

Protein tertiary structure

Three-dimensional arrangement of the whole protein chain; the shape that a protein chain assumes in space; includes fibrous and globular structures.

Proteolytic

Breaks down or hydrolyzes proteins.

Random coil

A protein secondary structure that exhibits no regular, ordered pattern.

Salting-in

Addition of a dilute salt solution to improve the dispersibility of a protein.

Salting-out

Addition of a concentrated salt solution to precipitate a protein.

Steric effects

Effects caused by the size and shape of the amino acids comprising the protein chain; spatial effects; for example, bulky amino acids can prevent a protein from folding upon itself in certain ways.

Water-binding capacity

The ability of a protein to bind water; this ability depends on the number of charged and polar groups along the protein chain.

Zwitterion

Contains a positively charged group and a negatively charged group within the molecule.